The present invention relates in general to lamps, and more specifically to tungsten halogen lamps or inert gas filled lamps having an improved glass envelope.
Tungsten halogen lamps have long been used in industry and provide certain advantages over conventional incandescent lamps. For example, the light emitted from the tungsten halogen lamp has a higher color temperature, is generally whiter, and the lamp can be made in much smaller dimensions than conventional lamps, while still producing an equivalent or greater amount of light. Furthermore, the intensity of the illumination from tungsten halogen lamps remains virtually constant throughout the life of the lamp. In addition, tungsten halogen lamps exhibit a significantly longer working life than conventional incandescent lamps. Although envelopes for tungsten halogen lamps of this type can be prepared from fused quartz and 96% silica glass compositions, these materials are difficult to form and lampwork due to their high working temperatures and their low coefficients of thermal expansion which necessitate special sealing techniques to introduce the lead wires into the lamps. Electrical leads must be introduced in the form of molybdenum foils with welded leads attached, thus achieving a means of supplying electrical current to the filament and at the same time achieving a hermetic seal. Such foil seals restrict the size reduction of a lamp and preclude an exhaust tube at the bottom of the lamp. Exhausting such lamps occurs through an exhaust tube located at the top end of the lamp and when tipped off leaves a knob of glass at the top of the lamp envelope, often referred to as top tipoff. This top tipoff becomes an optical obstruction when attempting to focus on the filament or to project light from a lens.
A distinct advantage for glass halogen lamps having envelopes made of conventional aluminosilicate glass is that the leads are introduced as lead wires and foils are not required. This then allows for considerable reduction in the size of the lamp. Further, the tipoff may occur at the bottom of the lamp between the lead wires, often referred to as the bottom tipoff. This then allows for the top end of the lamp to remain essentially optically clear for focusing on the filament and may contain a lens to project light. The ability to utilize the lamp from the end of the envelope is of considerable importance. For example, in spectroscopy, analytical chemistry, blood and serum analysis, medical diagnostics and clinical chemistry, end on use thru the clear hemispherical or lensed end has a significant positive effect on the design and use of instrumentation. The quartz lamp with its inherent top tipoff has a negative effect requiring the instrument designer to work around this optical obstruction. Further, the use of envelopes made of aluminosilicate glass allow for size reductions of the halogen lamp and allow the designer to design, with miniaturization in mind, a smaller instrument which is advantageous to hand held instrumentation and portable instrumentation. Therefore, there are distinct advantages in making halogen lamps from aluminosilicate glass.
For purposes of this invention aluminosilicate glass may be defined as glass with an SiO.sub.2 base, with a relatively high concentration by weight of Al.sub.2 O.sub.3 and with secondary composition oxides of BaO, CaO, and to a much lesser concentration the composition oxides of MgO and B.sub.2 O.sub.3. Commercially available aluminosilicate glasses fall into the broad composition by weight range set forth in Table I shown below:
TABLE I ______________________________________ SiO.sub.2 53-76% Al.sub.2 O.sub.3 9-18% B.sub.2 O.sub.3 0-6% CaO 3-13% MgO 0-9% BaO 5-24% ______________________________________
Such glasses have significant commercial value in the lamp industry for tungsten halogen regenerative cycle lamps as this family of glasses is characterized by a high thermal strain point, chemical compatibility, direct sealing to molybdenum lead wires and are easily worked.
A disadvantage of currently available aluminosilicate glasses for use in tungsten halogen lamps is the poor transmission of short wavelength light of less than about 400 nm. This puts the tungsten halogen lamp with glass envelopes made of these materials at a distinct disadvantage in that they do not achieve levels of near ultraviolet or ultraviolet (UV) light emission that can be derived from a tungsten halogen lamp with a quartz envelope of the same wattage and color temperature due to the fact that quartz glass has a higher transmission of light (transmittance) below 400 nm.
It can therefore be seen that there is a need in the field for tungsten halogen lamp envelopes made from glass compositions which can provide for both the working, sealing and performance properties associated with aluminosilicate glass systems, and which also exhibit increased transmission of UV light.